Split fan work gas turbine engine

ABSTRACT

One or more of the aft stages of the fan section of a gas turbine engine are rotated by the high pressure turbine of a gas generator and the remainder of the fan stages are rotated by a low pressure turbine downstream of the gas generator. This arrangement divides the fan work between the low and high pressure turbines to permit more efficient utilization of the total available turbine capacity, reduction in low pressure turbine workload, increased aft fan stage pressure ratio capability and greater flow and pressure ratio modulation potential for a split fan engine. The invention has a wide range of application for various turbofan configurations including separated and/or mixed flow turbofan engines with separated or close coupled fan sections.

BACKGROUND OF THE INVENTION

This invention relates to a gas turbofan engine and, more particularly,to a novel arrangement for the fan section thereof.

Considerable attention has been devoted to developing gas turbineengines with the high specific thrust characteristics of a turbojet orlow bypass turbofan at supersonic speeds which can also be configured toexhibit the lower specific thrust, low noise, and low fuel consumptioncharacteristics of a higher bypass turbo-fan at subsonic speeds in orderthat a very efficient mixed-mission aircraft may be developed. Suchengines are generally referred to as variable cycle engines.

Several design approaches for a variable cycle engine have been proposedincluding several modifications of mixed flow gas turbine engines. Thus,it has been proposed to vary the bypass ratio of a gas turbine engine byoperating it either as a mixed flow or a separated flow turbofan byselectively mixing or separating the bypass duct stream from the coreexhaust stream using diverter valves. It has also been proposed toincrease the flow modulation potential of a gas turbine engine bysplitting the fan into two sections, each in flow communication with aseparate concentric bypass duct.

One problem associated with all such prior art variable cycle engines isthe high workload imposed on the fan system to produce the desired highbypass ratios at low thrust flight. In order to supply sufficientrotational energy to produce these high fan flow rates, most prior artturbofan engines have utilized a multi-stage low pressure turbinedisposed downstream of the high pressure turbine of a gas generatorwhich operates at relatively high temperatures.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved gas turbine engine capable of operating over a wider rangeof thrust levels and bypass ratios with reduced low pressure turbinesize and lower operating temperatures.

It is also an object of this invention to provide a gas turbine enginein which the fan workload is shared by the high pressure turbine of thegas generator and a low pressure turbine disposed downstream of the gasgenerator.

These and other objects of the invention have been achieved in thepreferred embodiments of the invention wherein one or more of the aftstages of the multi-stage fan section of a gas turbofan engine arerotated by the high pressure turbine of the gas generator and theremainder of the fan stages are rotated by a low pressure turbinedownstream of the gas generator. This arrangement divides the fanworkload between the low and high pressure turbines to permit moreefficient utilization of the total available turbine capacity and areduction in low pressure turbine workload. Further, by directlycoupling the aft fan stages to the high pressure turbine rotor, the aftfan stages may be operated at increased rotational speeds to provide agreater pressure rise across the aft fan stages. The reduction of lowpressure turbine workload reduces the energy extraction potentialrequired of the low pressure turbine and resultant turbine coolingrequirements for the gas turbine engine of this invention, and furtherpermits the gas turbine engine of this invention to achieve a higherbypass ratio for a given low pressure turbine energy extractionpotential than is characteristic of prior art gas turbine engines inwhich the fan section is coupled only to the low pressure turbinesystem.

The invention has a wide range of application for various engineconfigurations and thus may be utilized in a split fan multiple bypassduct gas turbine engine, or in a single bypass mixed flow gas turbineengine with a split or close coupled fan sections. The inventionsignificantly increases the flow modulation potential of a split fanvariable cycle engine. Because the aft fan section is driven by the highpressure turbine, it may be operated at higher or lower rotationalvelocities than the front fan section which is driven by the lowpressure turbine thereby permitting increased flexibility in dividingthe engine airflow between the bypass ducts and gas generator andenabling a higher or lower pressure rise to be achieved across the aftfan section.

For the close coupled fan mixed flow engines, it may also be desirableto include an inter-fan bleed system between the low pressure turbinedriven front fan stages and the high pressure turbine driven aft fanstages, in order to bleed off excess front fan flow during low thrustflight.

The invention may also be utilized in combination with a downstreamdiverter valve, or a variable area mixer. These latter devices, whenutilized with the invention, provider a variable cycle gas turbineengine having a high degree of flow modulation potential which may beoperated efficiently through a broad range of engine thrust settings andbypass ratios.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood upon reading the followingdescription of the preferred embodiments in conjunction with theaccompanying drawings wherein:

FIG. 1 is a cross-sectional view of a split fan double bypass gasturbine engine incorporating this invention.

FIG. 2 is a cross-sectional view of a split fan mixed flow gas turbineengine incorporating this invention.

FIG. 3 is a cross-sectional view of a close coupled fan mixed flow gasturbine engine incorporating this invention.

FIG. 4 is a fragmented view of a portion of the gas turbine engine ofFIG. 3 is one mode of operation.

FIG. 5 is a fragmented view of a portion of the gas turbine engine ofFIG. 3 in a different mode of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a gas turbine engine 10 having anouter casing or nacelle 12, the upstream end of which forms an inlet 14sized to provide a predetermined design airflow to the engine 10.Disposed in the inlet 14 is a fan shown generally at 16 for receivingand compressing the airflow delivered to the inlet 14. The fan 16includes a front section 18 axially displaced from an aft fan section 20by an axial space designated generally at 22. This arrangement issimilar to the disclosed in U.S. patent application, Ser. No. 445,438filed by Bernard L. Koff et al on Feb. 25, 1974, now abandoned, andassigned to the same assignee at this invention. Front fan section 18includes a plurality of rotor blades 24 and 25, interspaced betweenvariable inlet guide vane 26 and variable stator vane 28. Similarly, theaft fan section 20 includes a rotor 30 and interspaced between aft fanvariable stator vanes 32 and 33.

Downstream of the fan 16 there is provided a gas generator showngenerally at 34. The gas generator includes a compressor 36 having arotor 38. Pressurized air entering the compressor 36 through a flowannulus 40 is compressed and then discharged to a combustor 42 wherefuel is burned to provide high energy combustion gases which drive ahigh pressure turbine rotor 44. The high pressure turbine rotor 44operates to extract energy from the high pressure gas stream exiting thecombustor 42, and to convert some of this energy into shaft horsepowerfor driving the rotor stages 38 of the compressor 36 through an upstreamextending driveshaft 48 connected for rotation with rotors 38 and 44.

Disposed downstream of the high pressure turbine rotor 44, in a positionto receive the flow of hot gases leaving the gas generator 34, is a lowpressure turbine shown generally at 50. The low pressure turbine 50includes a rotor section 52 having a plurality of rotor bladesinterspaced between stator blades 54. The low pressure turbine 50converts energy from the high pressure gases exiting the high pressureturbine into shaft horsepower and delivers this power to the rotors 24and 25 of the front fan section 18 through an upstream extendingdriveshaft 56. In accordance with one aspect of this invention, theshaft 48 driven by the high pressure turbine rotor 44 is extendedupstream of the gas generator compressor rotor 38 and interconnectedwith the rotor 30 so as to supply rotational energy to the spool 30 ofthe aft fan section 20.

In order to bypass a portion of the fan airflow around the core engine,there are provided two concentric bypass ducts. An outer duct, showngenerally at 58, is provided between the engine outer casing 12 and aintermediate casing 60. The upstream end 62 of the intermediate casing60 terminates in the annular space 22 between the front and aft fansections such that air entering the engine inlet 14 and compressed bythe front fan section 18 is thereafter divided between the outer bypassduct 58 and the aft fan section 20. The inner bypass duct showngenerally at 66 is formed between the intermediate casing 60 and theengine inner casing 68 circumscribing gas generator 34 and low pressureturbine 50. The upstream end 69 of the inner casing 68 terminates in theaxial space 70 separating the aft section 20 and gas generator 34 suchthat airflow compressed by and exiting from the aft fan section 20 isdivided between the inner bypass duct 66 and the gas generator 34.

In order to deliver propulsive force to the engine, a variable areaouter exhaust nozzle shown generally at 72 is provided to exhaust theflow in the outer bypass duct 58 to the ambient. The flow to the innerbypass duct 66 is mixed with the gas generator flow exiting the lowpressure turbine 50 in the region shown generally at 74 downstream ofthe low pressure turbine 50. For this purpose, a suitable mixer showngenerally at 76 is provided at the downstream end of the inner bypassduct 66 and upstream of outer bypass exhaust nozzle 72. The mixer 76 ispreferably of the variable area type as disclosed in U.S. application,Ser. No. 583,055 filed by J. Rundell et al on June 2, 1975. To furtherincrease thrust at high mach numbers, an after burner shown generally at78 may be provided downstream of the mixer 76. The intermixed flows fromthe gas generator 34 and inner bypass duct 66 are exhausted from asuitable variable area convergent divergent exhaust nozzle 77 formed atthe downstream end of the intermediate nacelle 60 and circumscribed bythe outer exhaust nozzle 72.

Prior art mixed flow engines have not operated efficiently throughout avariable thrust range because they experience significantly high inletdrag levels during low thrust flight. Typically, the inlet of a gasturbine engine is sized to be full at the maximum thrust of the engine.However, as engine thrust is decreased below the maximum thrust, theengine airflow demand is considerably less than the total airflowsupplied to the inlet. This excess airflow to the inlet causes inletspillage drag which has significantly increased the installed fuelconsumption of prior art mixed flow engines. The flow variabilityafforded by this invention permits the airflow to the engine inlet 14 tobe maintained at a matched design level throughout a wide range ofengine thrust levels, thereby avoiding the inlet spillage dragassociated with the prior art mixed flow engines and significantlyincreasing installed fuel consumption.

The variable cycle engine of FIG. 1 has a high degree of flowmodulation. The position of the inlet to the outer bypass duct 58 andthe inlet to the aft fan section 20 downstream of the front fan section18 and the position of the inlet to the inner bypass duct 66 and theinlet to the gas generator 34 downstream of the aft fan section combinewith variable inlet guide vanes and variable stator sections of thefront and aft fan sections to permit the total inlet airflow to bedivided between the outer bypass duct 58, the inner bypass duct 66, andthe gas generator 34 in varying proportions so that engine bypass ratiomay be varied over a wide range while maintaining the total engine inletairflow at the matched design level. More particularly, increasing theproportion of total inlet airflow which is directed to the outer andinner bypass ducts 58 and 66 respectively, while reducing flow throughthe gas generator 34, results in a higher engine bypass ratio.Similarly, decreasing the proportion of total inlet airflow to thebypass ducts 58 and 66, while increasing the air-flow to the gasgenerator 34, results in a lower bypass ratio.

By driving the aft fan rotor 30 with the high pressure turbine 44, thetotal workload of the fan 16 is divided between the high pressureturbine 44 and the low pressure turbine 50. This arrangement hasparticular utility in the low thrust high bypass flow mode of engineoperation. In such conditions, the bypass ratio is increased bybypassing a greater amount of the total inlet airflow around the gasgenerator through the outer bypass duct 58 and the inner bypass duct 66.This is generally accomplished by adjusting the variable inlet guidevanes and stator sections of the front and aft fan sections to maximizethe flow therethrough, and by adjusting the gas generator speed andvariable inlet guide vane 79 to minimize the flow to the gas generator.In this mode, the pressure rise in the front fan section 18 and aft fansection 20 requires a relatively large amount of energy to be extractedfrom the turbines supplying rotational energy to these fan sections.However, since the gas generator flow is significantly reduced for thislow thrust high bypass mode of operation, the energy required to drivethe compressor 36 of the gas generator is less than that which isrequired in high thrust conditions. Thus, there is an excess of energyextraction potential in the high pressure turbine. This excess highpressure turbine capacity is utilized to drive the aft fan section 20 inorder to reduce the workload required of the low pressure turbine 52.The resultant reduction in energy extraction potential required of thelow pressure turbine permits the low pressure turbine to operate atlower pressure ratios, thereby reducing the low pressure turbine workrequirements and/or permitting utilization of a fewer number of lowpressure turbine stages with resultant savings in cost, weight, andcooling flow. Thus, while low pressure turbine rotor 52 has beenillustrated as comprising three stages, it may be possible for someapplications to utilize a single stage for the rotor 52.

The engine of this invention also has greater flow flexibility thanprior art gas turbine engines of the split fan configuration. Becausethe aft fan section is directly coupled to the high pressure turbine, itmay be operated at different and higher rotational velocities than thosefan sections which are coupled to the low pressure turbine, therebypermitting increased flexibility in dividing the inlet airflow betweenthe outer and inner bypass ducts and the gas generator and enabling ahigher pressure rise to be achieved across the aft fan section 20.

In the high thust low bypass mode of engine operation, the variableinlet guide vanes and stators of the front and aft fan sections areadjusted to reduce the flow in the inner and outer bypass ducts andincrease the flow to the gas generator 34. Even though the energyextraction requirements for the high pressure turbine are increasedduring such flight conditions, the total energy extraction potential ofthe high pressure turbine is not exceeded. This is due in part to theincreased booster airflow from the aft fan section 20 to the gasgenerator 34 and the increased pressure rise across the aft fan section20 resulting from the higher rotational velocities of the high pressureturbine in this mode of operation.

Referring to FIG. 2, wherein like numbers refer to previously identifiedcomponents, therein is shown another embodiment for a gas turbine engineincorporating this invention. The engine of this embodiment utilizes asingle bypass duct 80 in lieu of the dual bypass duct arrangement ofFIG. 1. This arrangement is similar to that disclosed in copending U.S.application, Ser. No. 587,134 filed by J. Simmons on June 16, 1975. Thefan 16 of this embodiment is divided into a front and aft fan section 18and 20 respectively as in the embodiment of FIG. 1. An annular casing 82is provided intermediate the outer engine nacelle 12 and the innerengine casing 68. The intermediate casing 82 has its upstream end 84disposed in the axial space 22 between the front and aft fan sectionsand has its downstream end terminating approximately coplanar with theinlet 40 to the gas generator compressor 36. An annular diverter valveshown generally at 86 is secured to the downstream end of theintermediate casing 82 to provide a means to modulate the bypass ductflow. The diverter valve comprises an annular hinged flap 88 pivotallymounted to the downstream end of the intermediate casing 82 andextending downstream into the bypass duct 80. Suitable actuator meanswhich may comprise a linear actuator 90 having a control arm 92 indriving engagement with the annular hinged flap 88 is provided to rotatethe flap 88 about the downstream end of the intermediate casing 82. Thediverter valve 86 may be moved between an open position as shown in FIG.2 wherein the downstream end of the flap 88 abuts the inner wall of theengine outer casing 12 to a closed position as illustrated in phantom inFIG. 2, wherein the downstream end of the flap 88 abuts the outer wallof the engine inner casing 68 including all positions intermediate thefully opened and fully closed positions. In its fully open position, asillustrated in FIG. 2, the diverter valve 86 obtrudes the bypass duct asa point upstream of the aft fan section 20 such that the total airflowexhausted from the front fan section 18 is directed to the aft fansection 20. Thereafter, the air compressed by both fan sections isdivided between the bypass duct 80 and the gas generator 34. In itsfully closed position as illustrated in phantom in FIG. 2, the divertervalve 86 obtrudes flow from the aft fan section 20 to the inner bypassduct 80 such that the flow exiting the front fan section 18 is dividedbetween the bypass duct 80 and the aft fan section 20, and the totalflow exhausted from the aft fan section 20 is directed to the gasgenerator 34. This arrangement permits the pressure level of the innerbypass duct flow to be controlled directly as a function of therotational position of the diverter valve 86 and thereby permits a highdegree of flow moldulation without the use of a double bypass ductarrangement as in the embodiment of FIG. 1.

In the embodiment of FIG. 2, like the embodiment of FIG. 1, the aft fanspool 30 is coupled directly to the high pressure turbine 44 throughdriveshaft 48. The increased flow modulation resulting from the divertervalve 86 and the separate control of rotational speeds of the aft fansection 20 due to its connection to the high pressure turbine 44 permitsthe gas turbo-fan engine of FIG. 2 to operate over a wide range ofengine bypass ratios and thrust levels with the total inlet airflowmaintained at a matched design level throughout.

The fan configuration of this invention also has applicability to gasturbine engines in which the fan sections are closely coupled ratherthan split. Referring to FIGS. 3, 4 and 5 where like numbers refer topreviously identified components, therein is shown still anotherembodiment of a gas turbine engine incorporating this invention whereinthe front and aft fan sections 18 and 20 respectively are closelycoupled rather than separated by an axial spaced as in the embodimentsof FIGS. 1 and 2. The advantages resulting from sharing the low pressureturbine workload by connecting one or more of the aft rotor blades ofthe fan permit efficiency in the design of the low pressure turbinesystem for this embodiment as in the embodiments of FIGS. 1 and 2. Theenable the front fan airflow to be matched to the aft fan airflowthroughout widely varying bypass ratios and thrust levels, a conduit 94may be provided intermediate the front and aft fan sections so thatexcess front fan section airflow may be bled around the aft fan section20 to the bypass duct 96. The conduit 94 has its inlet 100 disposedintermediate the front end aft fan sections 18 and 20 respectively andits outlet 102 in flow communication with the bypass duct 96 at a pointdownstream of the inlets to the aft fan section 20 and gas generator 34.A suitable valve, shown generally at 104, is provided at the inlet 100to the conduit 94. The valve 104, as best seen in FIGS. 4 and 5,comprises a hinged flap 106 pivotally hinged at 108 to the upstream endof the conduit 94. A suitable actuator 110 having a control arm 112 indriving engagement with the flap 106 is provided to rotate the flap 106about the downstream end of the conduit 94. The flap 106 is of suitablecross-section such that it may be moved to a fully closed position asillustrated in FIG. 5 wherein flow in the conduit 94 is obtruded to afully open position as illustrated in FIG. 4 allowing a maximum lowthrough the conduit 94, as well as all position intermediate the fullyclosed and fully open positions. Thus, the amount of flow bled aroundthe aft fan section will be determined by the position of the valve 104such that under those flight conditions in which the total airflowexiting the front fan section 18 is in excess of that required by theaft fan section 20, the excess can be bled to the bypass duct 96 whilemaintaining the engine inlet airflow matched to the optimum designlevel. The actuator 110 which controls the flap 106 may be part of anysuitable inlet guide vane control system of the type well known in theart capable of controlling the rotational position of the valve 104 as afunction of gas generator speed. One such control system suitable forthis purpose is disclosed in U.S. Pat. No. 2,931,168.

Various changes could be made in the embodiments shown in FIGS. 1through 4 without departing from the scope of this invention. Thus,while single and double bypass embodiments have been illustrated,further flow variability could be achieved by increasing the number offan stages and bypass duct in flow communication therewith. In addition,exhaust nozzle systems other than those illustrated may be utilized toincrease the flow modulation potential. For example, the embodiment ofFIG. 1 might be modified by providing the outer exhaust nozzle 72 with aconvergent divergent section and extending the outer bypass nozzle 72downstream of the inner exhaust nozzle 77 such that the gases exhaustedfrom the outer bypass duct and the combined flows then exhausted fromthe common variable area converging diverging nozzle assembly. Aconfiguration of this type is disclosed in U.S. patent application, Ser.No. 583,056, filed by Johnson et al on June 2, 1975. Further, thevariable area mixer 76 may be replaced with an aft end diverter valve asdisclosed in the aforementioned Johnson et al application.

For simplicity in design, the number of variable geometry componentsillustrated in the above embodiments have been kept to the minimumnecessary to achieve a desired degree of flow variability; however, itis also possible to utilize other variable geometry components toprovide a greater degree of flow modulation. Thus, the compressor, lowpressure turbine, and high pressure turbines may be equipped withadditional variable stator blades or variable rotor blades, or avariable nozzle diaphragm may be provided intermediate the high pressureand low pressure tubines to achieve additional flexibility in flowmodulation without departing from the scope of this invention.

Therefore, having described preferred embodiments of the invention,though not exhaustive of all possible equivalents, what is desired to besecured by letters patent of the U.S. is claimed below.

What is claimed is:
 1. In a gas turbine engine having a compressor,combustor, high pressure turbine supplying rotational energy to thecompressor and low pressure turbine in serial flow relation, all beingcircumscribed by an inner engine casing and an outer nacelle spacedapart from the inner engine casing to define a bypass duct therebetween,the outer nacelle extending upstream of the inner engine casing todefine an inlet for the engine and downstream of the inner engine casingto define an exhaust system for the engine, there is provided:a frontfan section disposed in the inlet upstream of the inner engine casingand receiving rotational energy from the low pressure turbine, an aftfan section disposed intermediate the front fan section and compressor,receiving rotational energy from the high pressure turbine andcircumscribed by an intermediate casing which is spaced radially inwardfrom the outer nacelle to define a bypass duct around the aft fansection, and diverter valve means secured to the aft end of theintermediate casing for selectively distributing the flow compressed bythe front fan section between the aft fan section and the bypass ductaround the aft fan section and simultaneously therewith selectivelydistributing the flow compressed by the aft section between thecompressor and bypass duct around the compressor.
 2. The gas turbineengine of claim 1 wherein the diverter valve means is movable between afirst position in which the total gas flow exiting the front fan sectionis directed to the aft section and the gas flow exiting the aft fansection is divided between the compressor and the bypass duct around thecompressor and a second position in which the gas flow exiting the frontfan section is divided between the aft fan section and the bypass ductaround the aft fan section and the total gas flow exiting the aft fansection is directed to the compressor.
 3. The gas turbine engine ofclaim 2 wherein the diverter valve means comprises:a flap rotatablyhinged to the downstream end of the intermediate nacelle, and a linearactuator for rotating the flap about the aft end of the intermediatenacelle.